1. Field of the Invention
This invention relates to bitumen recovery from oil sand and more particularly to a treatment process for the removal of water and mineral from the product produced in a primary oil sand bitumen extraction process.
2. Description of the Related Art
Oil sands are a geological formation, which are also known as tar sands or bituminous sands. The oil sands deposits provide aggregates of solids such as sand, clay mineral plus water and bitumen—a term for extra heavy oil. Significant deposits of oil sands are found in Northern Alberta in Canada and extend across an area of more than thirteen thousand square miles. The oil sands formation extends from the surface or zero depth to depths of two thousand feet below overburden. The oil sands deposits are measured in billions of barrels equivalent of oil and represent a significant portion of the worldwide reserves of conventional and non-conventional oil reserves.
The oil sands deposits are composed primarily of particulate silica mineral material. The bitumen content varies from about 5% to 21% by weight of the formation material, with a typical content of about 12% by weight. The mineral portion of the oil sands formations generally includes clay and silt ranging from about 1% to 50% by weight and more typically 10% to 30% by weight as well as a small amount of water in quantities ranging between 1% and 10% by weight. The in-situ bitumen is quite viscous, generally has an API gravity of about 6 degrees to 8 degrees and typically includes 4% to 5% sulfur with approximately 38% aromatics.
The Athabasca oil sands are bitumen-bearing sands, where the bitumen is isolated from the sand by a layer of water forming a water-wet tar sand. Water-wet tar sand is almost unique to the Athabasca oil sands and the water component is frequently termed connate water. Sometimes the term water-wet is used to describe this type of tar sand to distinguish it from the oil-wet sand deposits found more frequently in other tar sand formations and in shale deposits including those oily sands caused by oil spills.
The extraction of the bitumen from the sand and clay-like mineral material is generally accomplished by heating the composition with steam and hot water in a rotating vessel or drum and introducing an extraction agent or process aid. The process aid typically is sodium hydroxide NaOH and is introduced into the processing to improve the separation and recovery of bitumen particularly when dealing with difficult ores. The hot water process is carried out in a vessel called a separator cell or more specifically a primary separator vessel (PSV) after the oil sand has been conditioned in the rotating drum.
The PSV process produces a primary bitumen froth gathered in a launder from the upper perimeter of the vessel; a mineral tailings output from the lower portion of the vessel and a middlings component that is removed from the mid-portion of the vessel. It has been found that production of the middlings component varies with the fines and clay content of the originating oil sand and is described more fully, for example in Canadian patent 857,306 to Dobson. The middlings component contains an admixture of bitumen traces, water and mineral material in suspension. The middlings component is amenable to secondary separation of the bitumen it contains, by introducing air into the process flow in flotation cells. The introduced air causes the bitumen to be concentrated at the surface of the flotation cell. The flotation of the bitumen in preference to the solids components permits the air entrained bitumen to be extracted from the flotation cell. Flotation of the air-entrained bitumen from the process flow is sometimes termed differential flotation. The air-entrained bitumen froth is also referred to as secondary froth and is a mixture of the bitumen and air that rises to the surface of the flotation cell. Typically, the secondary froth may be further treated, for example by settling, and is recycled to the PSV for reprocessing.
Further treatment of the primary bitumen froth from the PSV requires removal of the mineral solids, the water and the air from the froth to concentrate the bitumen content. Conventionally, this is done by the use of centrifuges. Two types of centrifuge systems have heretofore been deployed. One, called a solids-bowl centrifuge has been used to reduce the solids in froth substantially. To remove water and solids from the froth produced by a solids-bowl centrifuge; a secondary centrifuge employing a disk has been used. Disk centrifuges are principally de-watering devices, but they help to remove mineral as well. Examples of centrifuge systems that have been deployed are described in Canadian patents 873,854; 882,667; 910,271 and 1,072,473 (U.S. Pat. No. 4,383,914). The Canadian patent 873,854 to Baillie for example, provides a two-stage solid bowl and disk centrifuge arrangement to obtain a secondary bitumen froth from the middlings stream of a primary separation vessel in the hot water bitumen recovery process. The Canadian patent 882,667 to Daly teaches diluting bitumen froth with a naphtha diluent and then processing the diluted bitumen using a centrifuge arrangement.
Centrifuge units require an on-going expense in terms of both capital and operating costs. Maintenance costs are generally high with centrifuges used to remove water and solid minerals from the bitumen froth. The costs are dictated by the centrifuges themselves, which are mechanical devices having moving parts that rotate at high speeds and have substantial momentum. Consequently, by their very nature, centrifuges require a lot of maintenance and are subject to a great deal of wear and tear. Therefore, elimination of centrifuges from the froth treatment process would eliminate the maintenance costs associated with this form of froth treatment. Additional operating cost results from the power cost required to generate the high g-forces in large slurry volumes.
In the past, cyclones of conventional design have been proposed for bitumen froth treatment, for example in Canadian patents 1,026,252 to Lupul and 2,088,227 (U.S. Pat. No. 5,316,664) to Gregoli. However, a basic problem is that recovery of bitumen always seems to be compromised by the competing requirements to reject water and solids to tailings while maintaining maximum hydrocarbon recovery. In practice, processes to remove solids and water from bitumen have been offset by the goal of maintaining maximal bitumen recovery. Cyclone designs heretofore proposed tend to allow too much water content to be conveyed to the overflow product stream yielding a poor bitumen-water separation. The arrangement of Lupul is an example of use of off-the-shelf cyclones that accomplish high bitumen recovery, unfortunately with low water rejection. The low water rejection precludes this configuration from being of use in a froth treatment process, as too much of the water in the feed stream is passed to the overflow or product stream.
A hydrocyclone arrangement is disclosed in Canadian patent 2,088,227 to Gregoli. Gregoli teaches alternative arrangements for cyclone treatment of non-diluted bitumen froth. The hydrocyclone arrangements taught by Gregoli attempt to replace the primary separation vessel of a conventional tar sand hot water bitumen processing plant with hydrocyclones. The process arrangement of Gregoli is intended to eliminate conventional primary separation vessels by supplanting them with a hydrocyclone configuration. This process requires an unconventional upgrader to process the large amounts of solids in the bitumen product produced by the apparatus of Gregoli. Gregoli teaches the use of chemical additive reagents to emulsify high bituminous slurries to retain water as the continuous phase of emulsion. This provides a low viscosity slurry to prevent the viscous plugging in the hydrocyclones that might otherwise occur. Without this emulsifier, the slurry can become oil-phase continuous, which will result in several orders of magnitude increase in viscosity. Unfortunately, these reagents are costly making the process economically unattractive.
Another arrangement is disclosed in Canadian patent 2,029,756 to Sury, which describes an apparatus having a central overflow conduit to separate extracted or recovered bitumen from a froth fluid flow. The apparatus of Sury is, in effect, a flotation cell separator in which a feed material rotates about a central discharge outlet that collects a launder overflow. The arrangement of Sury introduces process air to effect bitumen recovery and is unsuitable for use in a process to treat deaerated naphtha-diluted-bitumen froth as a consequence of explosion hazards present with naphtha diluents and air.
Other cyclone arrangements have been proposed for hydrocarbon process flow separation from gases, hot gases or solids and are disclosed for example in Canadian patents 1,318,273 (U.S. Pat No. 4,944,867) to Mundstock et al; 2,184,613 (U.S. Pat No. 5,538,696) to Raterman et al and in Canadian published patent applications 2,037,856 (U.S. Pat. No. 5,400,569); 2,058,221 (U.S. Pat. No. 5,183,558); 2,108,521 (U.S. Pat. No. 5,221,301); 2,180,686; 2,263,691 (U.S. Pat. No. 5,938,803); 2,365,008 (U.S. Pat. No. 6,846,463) and the hydrocyclone arrangements of Lavender et al in Canadian patent publications 2,358,805, 2,332,207 and 2,315,596.